The present invention is generally related to a composite resin material used for dental restoration, and more particularly to a universal composite resin material suitable for all dental restorations incorporating a uniformly dispersed nanometer sized discrete particulate filler which provides high strength, improved wear resistance and gloss retention in clinical use.
In dentistry, practitioners use a variety of restorative materials in order to create crowns, veneers, direct fillings, inlays, onlays and splints. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a resin matrix. These materials may be dispersion reinforced, particulate reinforced, or hybrid composites.
Dispersion reinforced composites include a reinforcing filler of, for example, fumed silica having a mean particle size of about 0.05 xcexcm or less, with a filler loading of about 30%-45% by volume. Because of the small particle size and high surface area of the filler, the filler loading into the resin is limited by the ability of the resin to wet the filler. Consequently, the filler loading is limited to about 45% by volume. Due to the low loading, the filler particles are not substantially in contact with one another. Thus, the primary reinforcing mechanism of such dispersion reinforced composites is by dislocation of flaws in the matrix around the filler. In dispersion reinforced materials, the strength of the resin matrix contributes significantly to the total strength of the composite. In dentistry, dispersion reinforced composite resins or microfills are typically used for cosmetic restorations due to their ability to retain surface luster. Typically, these microfill resins use free radical-polymerizable resins such as methacrylate monomers, which, after polymerization, are much weaker than the dispersed filler. Despite the dispersion reinforcement, microfill resins are structurally weak, limiting their use to low stress restorations.
One example of a dispersion reinforced composite is HELIOMOLAR(copyright), which is a dental composite including fumed silica particles on the order of 0.05 xcexcm mean particle size and rare earth fluoride particle on the order of less than 0.2 xcexcm mean particle size. HELIOMOLAR(copyright) is a radiopaque microfill-type composite. The rare earth fluoride particles contribute to both flexural strength and radiopacity.
Particulate reinforced composites typically include a reinforcing filler having an average particle size greater than about 0.6 xcexcm and a filler loading of about 60% by volume. At these high filler loadings, the filler particles begin to contact one another and contribute substantially to the reinforcing mechanism due to the interaction of the particles with one another and to interruption of flaws by the particles themselves. These particulate reinforced composite resins are stronger than microfill resins. As with the dispersion reinforced composites, the resin matrix typically includes methacrylate monomers. However, the filler in particulate reinforced composites has a greater impact on the total strength of the composite. Therefore, particulate reinforced composites are typically used for stress bearing restorations.
Another class of dental composites, known as hybrid composites, include the features and advantages of dispersion reinforcement and those of particulate reinforcement. Hybrid composite resins contain fillers having an average particle size of 0.6 xcexcm or greater with a microfiller having an average particle size of about 0.05 xcexcm or less. HERCULITE(copyright) XRV (Kerr Corp.) is one such example. HERCULITE(copyright) is considered by many as an industry standard for hybrid composites. It has an average particle size of 0.84 xcexcm and a filler loading of 57.5% by volume. The filler is produced by a wet milling process that produces fine particles that are substantially contaminant free. About 10% of this filler exceeds 1.50 xcexcm in average particle size. In clinical use, the surface of HERCULITE(copyright) turns to a semi-glossy matte finish over time. Because of this, the restoration may become distinguishable from normal tooth structure when dry, which is not desirable for a cosmetic restoration.
Another class of composites, flowable composites, typically have a volume fraction of structural filler of about 10% to about 30% by volume. These flowable composites are mainly used in low viscosity applications to obtain good adaptation and to prevent the formation of gaps during the filling of a cavity.
Various methods of forming submicron particles, such as precipitation or sol gel methods, are available to produce particulate reinforcing fillers for hybrid composites. However, these methods do not restrict the particle size to at or below the wavelength of light to produce a stable glossy surface. In U.S. Pat. No. 6,121,344, which is incorporated by reference herein in its entirety, a resin-containing dental composite is described including a structural filler of ground particles having an average particle size of between about 0.05 xcexcm and about 0.5 xcexcm that has the high strength required for load-bearing restorations, yet maintains a glossy appearance in clinical use required for cosmetic restorations. Because the structural filler particles are ground, the particles are nonspherical, providing increased adhesion of the resin to the structural filler, thereby further enhancing the overall strength of the composite. Through the use of structural filler particles that are ground and that have an average particle size less than the wavelength of light, that is less than about 0.50 xcexcm, the dental composite exhibits the luster and translucency required for cosmetic restorations. Specifically, because the structural filler size is less than the wavelength of visible light, the surface of a dental restoration will reflect more light in some directions than in others even after wear of the composite by brushing. The visible light waves do not substantially interact with the structural filler particles protruding out of the surface of the composite, and therefore, haze is reduced and the luster of the surface is maintained even after substantial brushing. The particles are still large enough to reinforce the composite by the particulate reinforcement mechanism, so the restorations are also stress bearing. The number of larger particles, above 0.5 xcexcm in diameter, are also minimized to help produce the stable glossy surface.
In U.S. Pat. No. 6,121,344, fumed silica microfill particles having an average particle size less than about 0.05 xcexcm are added, preferably between about 1% by weight and about 15% by weight of the composite. The microfill particles contribute to dispersion reinforcement, fill the interstices between the larger structural filler particles reducing occluded volume, and provide a large surface area to be wetted by the resin to increase strength. The fumed silica microfill particles also contribute to the flow properties of the uncured resin. Fumed silicas are produced by hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. During this process, silicon dioxide molecules condense to form particles of size usually less than 50 nm. The particles then attach to each other and sinter together. Due to the nature of the flame process, a three-dimensional chain aggregate with a length of 200-300 nm forms. Further mechanical entanglement occurs upon cooling to give agglomerates. Attractive interactions between the surface silanol groups of the particles give thixotropic properties to liquids in which these fumed silicas are suspended. The fumed silicas are hydrophobically treated to make them compatible with resins employed, but still substantial interactions result from attractive interactions of the residual silanol groups that are not reacted. The particle-particle interaction prevents homogenous dispersion of the microfiller in the resin matrix and increases the viscosity of the suspension, which correspondingly decreases the workability of the composite paste. This places a limitation on the practical filler loading in fumed silica microfilled restorative composites. A high filler loading is desirable in dental restorations because the high loading provides a paste with improved handling properties over a paste with low filler loading. Moreover, higher loading gives a composite that after curing is lower in shrinkage, has a coefficient of thermal expansion better matching that of a natural tooth and has higher overall physical properties.
There is thus a need to develop a dental restorative composite that has minimal particle-particle interactions to afford high filler loading and lower viscosity contribution when suspended in methacrylate resin.
The present invention provides a resin-containing dental composite including a structural filler of ground particles having an average particle size of between about 0.05 xcexcm and about 0.5 xcexcm and a nanofiller having discrete, non-agglomerated particles of mean particle size less than about 100 nm. The dental composite of the present invention has the high strength required for load-bearing restorations, yet maintains a glossy appearance in clinical use required for cosmetic restorations. Because the structural filler particles are ground, the particles are nonspherical, providing increased adhesion of the resin to the structural filler, thereby further enhancing the overall strength of the composite. Through the use of structural filler particles that are ground and that have an average particle size less than the wavelength of light, that is less than about 0.50 xcexcm, the dental composite of the present invention provides the luster and translucency required for cosmetic restorations. The discrete, non-agglomerated nanofill particles contribute to dispersion reinforcement, fill the interstices between the larger structural filler particles reducing occluded volume, and provide a large surface area to be wetted by the resin to increase strength. Moreover, particle-particle interactions are minimized, thereby allowing for high filler loading and lower shrinkage upon curing.
The present invention, in a preferred form, is a dental restorative composite which includes a ground structural filler having a mean particle size between about 0.05 xcexcm and about 0.50 xcexcm and a nanofiller having a mean particle size less than about 100 nm in a curable resin, preferably a polymerizable resin containing methacrylate monomers. Curing of the composite may be achieved by mixing two paste components containing a catalyst and accelerator, respectively, or by a photopolymerization process wherein the resins are cured when exposed to actinic radiation, such as blue visible light. Photopolymerizable resins containing monomers other than methacrylates may be used in the present invention, as may be appreciated by those skilled in the art, such as cationically photocurable oxiranes, for example. The dental composite is applied to teeth by the dental practitioner and, for example, exposed to a visible light source to cure the resin. The cured resin has a flexural strength higher than 100 MPa which allows for the use of the resin in stress bearing applications.
To provide ground structural filler having a mean particle size of less than 0.5 xcexcm, an extensive comminution step is required. Comminution may be performed in an agitator mill, and preferably an agitator mill designed to minimize contamination, such as that described in U.S. Pat. No. 6,010,085, incorporated herein by reference in its entirety. Alternatively, comminution may be performed in a vibratory mill, and preferably in a vibratory mill designed to minimize contamination, such as described in U.S. Pat. Nos. 5,979,805 and 6,098,906, each incorporated herein by reference in its entirety. Comminution deagglomerates the structural filler particles by separating particles from clusters, decreases the size of the structural filler particles, eliminates large particles by breakage and increases the specific surface area of the structural filler particles by producing a large quantity of very fine particles. Size reduction with an agitator mill or vibratory mill occurs due to a combination of impact with the milling media, abrasion with the milling media and attrition of the particles.
Structural fillers suitable for use in the present invention include barium magnesium aluminosilicate glass, barium aluminoborosilicate glass (BAG), amorphous silica, silica-zirconia, silica-titania, barium oxide, quartz, alumina and other inorganic oxide particles. The mean particle size of the structural filler is limited to less than the wavelength of light to prevent the structural filler from decreasing surface gloss after substantial brushing. However, it is expected that as the particle size is reduced below about 1 xcexcm the strength needed for load-bearing restorations demises due to increasing occluded volume of resin. Currently, it is believed that a mean particle size between about 0.05 xcexcm and about 0.5 xcexcm provides the best balance between optical and structural properties.
Nanofillers suitable for use in the present invention include powders with particles that are not aggregated or substantially agglomerated so as to minimize particle-particle interactions. The discrete particles have a mean particle size less than 100 nm. By xe2x80x9cdiscrete particles,xe2x80x9d there are included weakly agglomerated particles having an agglomerated average size less than 100 nm. For example, Nanomaterials Research Corp., Longmonte, Colo., manufactures an aluminosilicate powder having a mean particle size of about 80 nm and a 1:4 molar ratio of alumina to silica. This nanofiller has a refractive index of 1.508. The powder is produced by a thermal quench process using a plasma torch for vaporization, such as described in U.S. Pat. Nos. 5,984,997; 5,851,507; and 5,788,738, each incorporated by reference herein in its entirety. The powder produced by the plasma or thermal quench process using a gas phase precursor includes discrete, non-agglomerated particles of narrow particle size distribution.
By way of further example, Nanophase Technologies Corp., Romeoville, Ill., manufactures gamma alumina powders having mean particle sizes less than 20 nm, as well as a powder having a mean particle size of 38 nm. This nanofiller has a refractive index of about 1.71. The powder is produced by a physical vapor synthesis process, such as described in U.S. Pat. Nos. 5,874,684; 5,514,349; and 5,460,701, each incorporated by reference herein in its entirety.
The nanofiller particles may be surface treated, for example with gamma methacryloxypropyltrimethoxy silane (MEMO). The nanofiller comprises at least about 0.01% by volume of the dental composite, more advantageously about 1-15% by volume, and most advantageously about 5-12% by volume.
Generally, the nanofiller should have a refractive index similar to that of the resin. Resins typically have a refractive index of about 1.48-1.55. Thus, the nanofiller should have a refractive index in the range of about 1.48-1.6. However, it is believed that for nanofillers of 20 nm particle size or less, the refractive index may vary from that of the filler without negatively affecting the optical properties of the dental composite. Thus, for gamma alumina nanofiller, the particle size is preferably 20 nm or less due to its relatively high refractive index.